Polyesters, particularly polyethylene terephthalate, are widely used as fibers, films and other shaped articles because of having superior physical and chemical properties. However, contrary to such superior properties it is also known that there occur undesirable troubles such as the deterioration of tile working efficiency and of the product value because of poor process passing property in a film or other shaped article-forming process for obtaining the aforementioned shaped articles and poor slipperiness in the handling of products.
In connection with such problems, there have been proposed many methods for improving the surface slipperiness of products by incorporating fine particles in polyesters to impart moderate concave and convex shapes to the surfaces of the shaped articles, and some of the methods have been adopted practically. For example, there is known a method (e.g. Japanese Patent Laid Open Nos. JP55-133431A and JP57-125247A) wherein inert inorganic particles such as the particles of silicon oxide, titanium dioxide, calcium carbonate, talc or kaolinite, or fine particles of organic polymers such as benzoguanamine-formaldehyde resin, polytetr; afluoroethylene-hexafluoropropylene copolymer or polyphenyl ester resin, are added to a polyester preparing reaction system.
However, in the method of adding inert inorganic particles, the affinity for polyester is generally poor, so when formed into a film for example, voids are formed in the vicinity of the particles. Besides, coarse particles are often mixed into the polyester, and even if pulverizing or classifying operations are performed for removing such coarse particles, the incorporation of the coarse particles is unavoidable. If such voids are formed or coarse particles are present, then in the case of films for which transparency is required, such as, for example, films for printing plates and microfilm, the transparency will be markedly deteriorated, and in the case of films for capacitors, a bad influence will be exerted on electrical properties. Further, in the case of a film for magnetic tapes, electro-magnetic conversion properties will be deteriorated, or drop-out will occur, thus resulting in impairment of the film quality.
On the other hand, according to the method of adding fine particles of an organic polymer, the affinity for polyester is generally good, but it is difficult to obtain uniform and fine particles, and even if such fine particles are obtained, their dispersibility in the polyester is poor. For example, the particles will agglomerate in the polyester. Thus, incorporation of coarse particles is unavoidable.
Recently, with respect to films for which transparency is required, films for capacitors and films for magnetic tapes, a higher quality has been required, and a film superior in slipperiness, having a uniform surface and containing particles superior in the affinity for polyester has been desired.
It is an object of the present invention to eliminate the above-mentioned disadvantages of the conventional particles adding methods.
More particularly, the objects of the present invention are to provide a polyester composition wherein crosslinked polymer particles maintain a uniform monodisperse state even when held in a melted state for a long time, said polyester having specific functional groups, to provide a process for preparing the same and to provide a polyester film superor in all of slipperiness, surface unifomity, transparency and abrasion resistance.